In study on the function of substance P and on the effect of drugs on its distribution, the possibility that a pool of substance P precursor exists must be considered. The chick embryo dorsal root ganglion contains a molecular species of high molecular weight immunoreactivity which could function as a precursor in the formation of substance P. The content of this possible precursor is regulated by treatment of ganglia with nerve growth factor. Substance P, and its apparent precursor, have also been found in superior cervical ganglia, probably located in interneurons. In order to carry out pulse chase experiments to demonstrate the nature of the relationship between this possible precursor and substance P, cultures of dissociated neurons from chick embryo dorsal root ganglia have been developed. These cultures are also useful for examining the mechanism of action of drugs such as capsaicin and for studying the question of the coexistence of two transmitters such as substance P and serotonin in the same neuron. In parallel similar studies are being carried out in the brain.